What is the approximate molality of a 68 percent by mass aqueous nitric acid solution?
Answer and explanation
Correct answer: 33.7 mol kg−1
Molality uses kilograms of solvent, not kilograms of the entire solution. Choose 100 g solution: 68 g is HNO3 and 32 g is water. Moles of HNO3 = 68/63 ≈ 1.079 mol, while solvent mass = 32 g = 0.032 kg. Hence molality = 1.079/0.032 ≈ 33.7 mol kg−1, so option C is correct. Dividing by 0.100 kg would incorrectly use solution mass and produce a different concentration.
Frequently asked questions
What is the correct answer to this question?
33.7 mol kg−1
Why is this the correct answer?
Molality uses kilograms of solvent, not kilograms of the entire solution. Choose 100 g solution: 68 g is HNO3 and 32 g is water. Moles of HNO3 = 68/63 ≈ 1.079 mol, while solvent mass = 32 g = 0.032 kg. Hence molality = 1.079/0.032 ≈ 33.7 mol kg−1, so option C is correct. Dividing by 0.100 kg would incorrectly use solution mass and produce a different concentration.
Which subject and chapter does this question cover?
This is a Class 11 Chemistry question. Chapter: Some Basic Concepts of Chemistry. Topic: Mole concept and molar mass.
